For the successful realization of advanced semiconductor devices, novel materials are expected to be introduced in future IC technology nodes (45 nm node and beyond) such as high-k dielectric elements and metal gate electrodes. Several integration schemes have been proposed and are currently practiced in order to build functional devices containing new gate dielectric materials such as HfO2, having a higher dielectric constant than SiO2. Also, it has been demonstrated that the use of a thin (about 2 to about 10 Å) oxide capping layer (e.g., containing Group IIA, IIIA and IIIB elements such as La2O3, MgO, SrO, Y2O3, Al2O3 or BaO) on top of HfO2 enhances the threshold voltage tunability of the device with reduced gate leakage and adds extra channel control, formerly afforded by the low-k dielectric material. In particular, a La2O3 layer as a cap on an HfO2 layer provides optimum performance and threshold voltage control.
In order to define nFET and pFET functional areas at the chip level, selective removal of the ultrathin (sub-1 nm) La2O3 layer is required. Lithographic patterning with a ca. 2000 Å thick photoimageable layer (photoresist) is used towards that end, optionally utilizing an intermediate layer between La2O3 and photoresist consisting of a ca. 500 Å thick developable Bottom AntiReflective Coating (dBARC) for resist adhesion and reflectivity control purposes. Wet etch of the exposed La2O3 regions to expose the substrate electrode (HfO2 layer) is performed with diluted hydrochloric acid (HCl), while the regions masked by the patterned resist/BARC areas remain intact. The HCl wet etchant concentration is adjusted to prevent the attack/removal of the HfO2 layer. Finally, the photoresist/BARC layers are removed with a N2H2 plasma dry etch process.
Two fundamental problems affect the reliability of the sequence described above. First, the contact between aqueous photoresist developer (TMAH 0.26N) and the surface of the lanthanide (La2O3) layer introduces a slight dissolution of the lanthanide layer, thus affecting the reworkability of the entire patterning scheme. Ideally, no La2O3 should be removed during the photoresist patterning step. Second, the utilization of N2H2 plasma for the removal of thick resist and dBARC layers is prone to defectivity formation in the form of residual strings, blobs, stains and particulates that originate from the interaction between the plasma and the thick organic material layers.
Therefore, the use of an ultrathin adhesion layer that replaces the thick dBARC is required to prevent contact of the resist developer with the La2O3 surface and to eliminate a source of post-etch defects, while still providing good adhesion between the imaging layer and the La2O3 surface.